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An early morning rockslide in Ohio sent towering boulders crashing onto a roadway near the state line — and highlighted an underappreciated and under-researched class of natural disaster.

Several hulking chunks of rock slid onto Route 7 near the town of Chesapeake in Lawrence County, Ohio, just before 7:30 a.m. today (Feb. 26) according to WSAZ News. Like many landslides, this one was both predictable and mysterious, according to Mika McKinnon, a landslide researcher, geophysicist and science communicator from British Columbia.

"Landslides happen anywhere you have slopes and gravity," McKinnon told Live Science. But their exact trigger — whatever it is that makes the rocks break and slide at that very moment and not 10 seconds sooner or later — is rarely known. [Lessons from the 10 Worst Engineering Disasters in US History]

Wherever you are on the globe, you have about a one-in-a-million chance each year of dying in a landslide, McKinnon said. That's not a geological rule. It's a human one. Construction engineers try to mitigate the likelihood of landslides onto roadways by doing such things as putting up rockfall netting on steep roadcuts, driving bolts into steep hillsides or pre-emptively blasting away cliffs over high-traffic areas to remove precariously positioned debris.

Hong Kong spends a little more money on mitigation than most places, reducing risk there to a 1 in 10 million chance of death, McKinnon said; the Philippines spends less, so the chance of people dying in a mass Earth movement there is 1 in 10,000. But in most cases, mitigation to reduce risk to one in a million is considered successful engineering, McKinnon said.

For the most part, it's pretty obvious where landslides are likely to happen: Anywhere where a slope is overly steep, especially when the slope is weakened by snow or rain. Deforestation can increase the odds, McKinnon said, because vegetation helps knit soil and rock together; early spring is prime time for landslides and rockfalls, because the freeze-thaw cycle opens up new cracks and widens pre-existing ones. 

But beyond these generalities, it's hard to predict precisely when and where a landslide will occur. That's partially because it's not practical to monitor every slide-prone hillside, McKinnon said, and partially because landslide research is funded at lower levels than other geological disaster research. For example, in the 2017 U.S. Geological Survey budget, $149,701 was requested for natural-hazard monitoring. Of that amount, $4,054 was earmarked for landslides.

The impact of landslides is often camouflaged because landslide deaths are often lumped in with a wider natural disaster, McKinnon said. If an earthquake triggers a landslide that swallows a village, for example, the resulting deaths get attributed to the quake, not the slide itself.

Even unstable hillsides can be stubbornly unpredictable, remaining in a precarious state for months, years, and even centuries — until one day, something snaps.

"It could be rainfall, it could be one last little bit of freeze thaw, it could be the vibrations from a semitruck, it could be a butterfly flaps its wings and lands on a branch nearby and that was it," McKinnon said.   

In Ohio, the rainy weather and flooded river (Route 7 follows the Ohio River's path along the Ohio-West Virginia state line) certainly contributed to the hill's instability, McKinnon said. But "the exact trigger for why this happened now as opposed to next week or next month or 100 years from now — we don't know," she said.

Whatever the cause, the Ohio State Highway Patrol now has its work cut out for it. Though no one was injured in the rockfall, officials told news agencies it could be several days before the road is open again, because the boulders will have to be broken up before they can be moved.

Original article on Live Science